The invention relates to a process for purifying waste furnace gases from melting, remelting and incinerating plants, as well as to the utilization of the filter dusts produced thereby.
The invention finds application particularly for the production or refining of nonferrous metals, the filter dusts produced being returned to the melting process.
It is well known that harmful substances, contained in the crude gas, can be removed from the waste gases by adsorption on additives or by chemical reaction with the additives. In this way, the waste gases are freed from materials including dusts, sulfur dioxide, HCl and/or HF.
For example, wet, semi-dry and dry processes are known, for which, for example, burnt lime, calcium hydroxide or alkali compounds are used as additives.
It is a disadvantage of the known methods that the elimination rate of the dioxins and furans frequently does not satisfy requirements.
The resulting filter dusts are usually dumped as garbage.
For metal refining processes, metal or metal scrap, melting aids, such as blanketing salts, and energy are supplied. Metal, slags, such as salt slags, and waste gases containing harmful substances and dust leave the production process.
The resulting slags either are refined to recover valuable materials or dumped as garbage. The waste gases are freed from dusts, sulfur dioxide, HCl and HF. Usually, methods are used for this purpose for which such materials as burnt lime or calcium hydroxide are used as adsorbents. The resulting filter dusts are dumped as garbage.
Different types of furnaces, which are heated electrically or with gas or oil, are used to produce cast aluminum alloys. The rotating drum furnace is widespread. It is suitable particularly for melting contaminated aluminum waste, shavings and other materials in small pieces, particularly however charge materials with a low yield.
Melting aluminum-containing scrap (aluminum waste, shavings, waste metal, etc.) in the rotating drum furnace usually takes place under a closed salt blanket. The task of the salt is to prevent oxidation of the aluminum, to bind the nonmetallic impurities of the metal-containing charge and to facilitate the detachment of oxide layers from the base metal. The salt, from which the salt blanket is formed, consists of about 70% NaCl, about 30% KCl and approximately 5% flux. The process temperatures are between 600.degree. C. and 900.degree. C., depending on the alloy. Fossil fuels are used as energy carrier.
Aside from NaCl and KCl, aluminum, in the form of AlCl.sub.3 and AlF.sub.3, also evaporates at the temperatures existing during the melting process, the latter being decomposed in the waste gas stream to HCl and HF. Aside from these acidic components, the waste gas stream also contains SO.sub.2, it being possible for the composition of the components of the waste gas to fluctuate depending on the materials that are to be melted and on the heating medium. Furthermore, sublimed alkali chlorides and fluorides, aluminum oxide and small amounts of heavy metals, aluminum in metallic form and free and organically bound carbon in the form of, for instance, dibenzodioxins and dibenzofurans (PCDDD/F) are contained in the waste gas. The waste gas, as well as the resulting salt slags must be freed from the harmful substances.
Technological solutions for the purification of the salt slag are known. For example, the salt slag can be purified by the dissolving method in such a manner, that the purified mixed salt can be used once again as a blanketing salt in the rotating drum furnace. The insoluble sediments (alumina residues), remaining after the dissolving process, are processed further.
The waste gases, which contain components that contaminate the environment, are presently freed from undesirable components in waste gas purification systems.
Usually the waste gases are reacted in a dry process at about 100 to 350.degree. C. with Ca(OH).sub.2 as absorbent, free chlorine, HCl, HF and SO.sub.2 being reacted to form CaCl.sub.2, CaF.sub.2 and CaSO.sub.4. The dioxins and furans are bonded adsorptively.
After the gas-dust mixture has been cooled to about 90 to 200.degree. C., the dust is deposited in fabric filters. It has been customary up to now to dump the dusts as garbage. From economic and ecological points of view, efforts must be made to work up these filter dusts, which are obtained in an amount of approximately 20 to 70 kg/t of secondary aluminum. The object of the invention is to provide a process for decreasing the dioxin and furan content in flue gas, especially in the crude gas produced during processing of non-ferrous metals, with simultaneous optimal purification of the waste gases.